我尝试在 Texshop 中将多行与 Xelatex 结合使用，但这样做时会发生这种情况。这是一份最小文档：
>\documentclass{amsart}
\usepackage{amsmath}
\begin{document}
\begin{multiline}
a+b+c+d+e+f=g\\
l+m=n
\end{multiline}
\end{document}
控制台输出为：
This is XeTeX, Version 3.14159265-2.6-0.99996 (TeX Live 2016) (preloaded format=xelatex) restricted \write18 enabled. entering extended mode (./mwe.tex LaTeX2e <2016/03/31> Babel <3.9r> and hyphenation patterns for 83 language(s) loaded. (/usr/local/texlive/2016/texmf-dist/tex/latex/amscls/amsart.cls Document Class: amsart 2015/03/04 v2.20.2 (/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsmath.sty For additional information on amsmath, use the `?' option. (/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amstext.sty (/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsgen.sty)) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsbsy.sty) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsopn.sty)) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/umsa.fd) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/amsfonts.sty)) (./mwe.aux) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/umsa.fd) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/umsb.fd) ./mwe.tex:5: LaTeX Error: Environment multiline undefined. See the LaTeX manual or LaTeX Companion for explanation. Type H <return> for immediate help. ... l.5 \begin{multiline}
我怀疑自己做了一些愚蠢的事情，但我很高兴有人告诉我是什么。谢谢